Modulation system



Patented July 7, 1942 MODULATION SYSTEM Robert J. Ehret, Collingswood,and John L. Barnes, Merchantville, N. J., assignors to Radio Corporationof America; a corporation of Delaware Application May 22, 1940, SerialNo. 336,550

2 Claims.

This system relates to frequency modulation systems, and moreparticularly to an improved frequency modulator of the type in which thecontrol frequency of a piezo-electric crystal element is varied.

One of the principal problems which confronts the designer of afrequency modulated transmitter is that of providing effective frequencymodulation without sacrificing the necessary stability of the average orunmodulated carrier frequency. This is particularly true in the designof high frequency transmitters which utilize a relatively low frequencyoscillator and series of frequency multipliers, since undesired changesin the average oscillator frequency are multiplied by the same factor asthe desired step-up of the average frequency.

The use of a modulated crystal controlled low frequency oscillator hasbeen proposed, in which the control frequency of a piezo-electriccrystal is modulated, by varying the air gap between the crystal elementand one of the electrodes.

crystal by an electro-magnet adjacent to the diaphragm which isenergized by the modulating voltage. The crystal is connected in aconventional oscillator circuit and stabilizes the average frequency ofthe oscillator in the usual manner.

Variations in the air-gap,however, produce vari- 3 ations in theoscillator frequency which can be utilized in accordance with wellestablished'practice toenergize or excite a transmitter, for example.Such an arrangement is described in Patent No. 1,933,735, issuedNovember 7, 1933, to A.

Hund, for example.

It has been found that rather small variations in the oscillatorfrequency are produced by the systems of the prior art. This may beexplained by the fact that the diaphragm does not vibrate as a whole :atall frequencies but breaks up into different modes of vibration fordifferent frequencies in a manner which is similar to the production ofstanding waves on a vibrating string.

Thus, when one portion of the diaphragm is moving toward the crystalanother portion may be moving away from the crystal, so that theeffective change in the gap is minimized.

nent magnet as a core.

In addition it is well known that a magnetically operated diaphragmproduces double frequency components unless a fixed magnetic flux isemployed to pull or bias the diaphragm away from its neutral position.This is, of course, the reason that the electromagnets of all telephonereceivers, headphones, and the like, employ a perma- If such anarrangement be utilized to eliminate frequency doubling in the crystalmodulator it will be appreciated that the pull on the diaphragm willgive it a curved shape so that the effective air gap .at the center isincreased. To obtain the maximum frequency variation for a givendiaphragm movement, however, it is essential that the spacing betweenthe crystal and the diaphragm be as small as possible. It will beappreciated, therefore, that for a given diaphragm movement it is notpossible to obtain as great a change in frequency with the biasedmagnetic diaphragm as with one which is not so biased, and yet thebiasing is necessary to reduce distortion.

Accordingly it is the principal object of this invention to provide afrequency modulated oscillator having a stable average frequency. Otherobjects include the provisionof an improved crystal modulator of thevariable air gap type; the provision of improved means for driving themovable electrode of a crystal modulator; the provision of a light,economical crystal modulator suitable for use in portable equipment; andthe provision of a variable air gap crystal modulator of improvedoperating characteristics.

In brief, the foregoing objects are accomplished, in accordance with thepresent invention by providing the crystal with a substantiallyinflexible spaced electrode which is mounted on an electromagnetic orelectrodynamic motor mechanism, the electrode being movable bodilytoward and away from the crystal. Such an electrode need not be rigidlysupported at the edges, as does a stretched diaphragm, and consequentlywill present less impedance to the driving mechanism. Its surface willnot be distorted by wave patterns, nor is it necessary to distort theelectrode to overcome second harmonic generation. As a result the spacedelectrode may be smaller than the diaphragm, and the unmodulated Ornormal air gap may be reduced, thus increasing the modulating range ofthe device. In another embodiment of this invention the drivingmechanism for the spaced electrode is another piezo-electric element ofthe pick-up type, such as a Rochelle salt crystal, or the like.

This invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing, and itsscope is indicated by the appended claims. Similar reference numeralsrefer to similar elements throughout the drawing.

Referring to the drawing, Figure 1 is a schematic drawing of anelectromagnetic modulator; Figure 2 is a section view of a modulatorassembly, Figure 3 is a schematic drawing of an electrodynamicmodulator; and Figure 4 is a schematic drawing of 1a piezo-electricmodulator.

Referring to Fig. 1, a quartz crystal 5 is mounted on a base plate 1which may be any conventional crystal holder. A spaced electrode 9 ispositioned adjacent the upper face of the crystal, and is rigidlymounted on and supported by an arm I I which is, in turn, connected to asoft iron magnetic armature l3, so that movement of the armatureproduces a like movement of the electrode 9 in a direction perpendicularto the surface of the crystal.

The magnetic armature I3 is pivotally mounted at a point l5 intermediateits ends between the opposed faces of a pair of U-shaped pole pieces I!and I9 which are oppositely magnetized by a permanent or electromagnet2|. A deflecting coil 23, shown in cross-section between the pole piecesI! and 19, produces a magnetic field which coincides with the armatureaxis so that the armature is magnetized by currents flowing through thecoil. It will be appreciated that the effect of the two similar poles ofthe. two pole pieces will have a cumulative pull on the magnetizedarmature to cause it to rotate in one direction or the other, dependingon the direction of the current in the deflecting coil. The pivotalmounting I5 is suiliciently stiff to prevent excessive movement of thearmature. Consequently, the spaced electrode 9 will vibrate at afrequency corresponding to the frequency of the energizing currentapplied to the input terminals.

The spaced electrode 9 is grounded and the base plate 1 connected to thegrid of a thermionic oscillator tube 25, or the connections may bereversed, if desired. The anode of the oscillator tube 25 is connectedto an oscillatory circuit 21 and a source of anode potential 29 in theconventional manner. A grid leak 3| supplies operating bias for thetube.

It has been found that increased frequency modulation will result from agiven spacin and -movement of the top electrode when the crystal isconnected in parallel with a fixed capacitor 33. This result isapparently contrary to that which would normally be expected, since theeffective change of capacity is reduced by the shunt capacity. The valueof this capacitor may be as high as several hundred micro-microfaradsfor a crystal operating on a fundamental of 7 inc, but its actual valuefor maximum frequency shift will depend upon the particular oscillatorcircuit.

The cross-sectional drawing of a crystal modulator unit illustrated inFig. 2 shows the constructional details of an actual devicecorresponding to the schematic illustration of the preceding figure. Thecrystal 5 is mounted on a base plate 1 and clamped at the edges by meansof a retaining ring 35. By the term base plate is herein meant the fixedelectrode of the crystal, which may comprise a conductive mounting platewhich is also an electrode, or a conductive material plated on thecrystal which is then mounted on a non-conductive support. The baseplate 1 is mounted on an adjusting screw 31 for setting the air gapbetween the crystal and the spaced electrode 9. The screw 31 is threadedinto a collar 39 which is mounted in the bottom of a cuplike porcelaincontainer 4| which forms the main body of the crystal holder. Electricalconnection to the fixed electrode is made by means of a pin 43 set inthe porcelain, and connected to the collar 39 by means of a lead 45.

The top of the porcelain cup is covered by an apertured plate 41,through which extends the driving member H which connects the magneticarmature 13 to the spaced electrode 9. The driving member is held inposition by a stiff wire 49, one end of which is connected at rightangles to the driving member II at or near the electrode, the other endbeing connected to the top plate 41. The wire 49 functions to maintainthe electrode parallel to the crystal, throughout its range of movement.In the absence of such a guiding wire the electrode 9, arm H andarmature l3 would tend to rotate about the axis I5 and the electrodewould not remain parallel to the crystal, which is an undesirablecondition. This lead or wire 49 also provides an electrical connectionto the spaced electrode of the crystal. A mounting pin 5| iselectrically connected to the top plate 41 by a screw 53. Variousmaterials may be used in the construction of these posts, but preferablymaterials having a low temperature coefficient should be used. Metals ofdiffering coefficients may also be utilized to compensate for undesiredexpansion of the elements due to changes in temperature.

The top plate of the crystal holder is fixedly connected to the U-shapedpole piece I! of the driving mechanism. For convenience, the deflectingcoil is wound in two serially connected sections and 51 separated byinsulation to provide a space for the armature pivot which is, forexample, a resilient spring strip 59, the lateral cross section of whichis shown. The pole pieces I! and I9 are mounted on supporting members 6|and 63 which are fastened to the ends of a highly magnetized permanentmagnet 65.

The arrangement illustrated in Fig. 3 utilizes an electrodynamic drivemechanism similar to that employed in loudspeakers. It comprises apermanent magnet 61 and a voice coil 69 mounted in the intense magneticfield of the magnet by means of a flexible spider H. The usual cone isreplaced by a metallic electrode 9 which is adjacent the surface of apiezo-electric crystal element 5. The crystal is mounted on a base plateI, as before. It is to be understood that the crystal holder may be ofthe type illustrated in detail in Fig. 2, although, for the sake ofsimplicity, constructional features well known to those skilled in theart have been omitted. It' will be understood that the crystal may beconnected to any oscillator to control its average frequency, whichfrequency will be modulated by a signal voltage applied to the wire coilthrough the input terminals.

A unique driving mechanism is illustrated in Fig. 4, wherein onepiezo-electric element is used to control the frequency of anoscillator, while a second-piezo-electric element is used to drive thespaced electrode of the first, and thus modulate the oscillatorfrequency.

The first piezo-electric element 5 is mounted in the usual manner on abase plate 1 which is, for example, insulatingly supported on a member13. Immediately above the first crystal, a second piezo-electric element15 is mounted. This element may be of the type commonly used invibration pickup devices and loudspeakers such a a quartz or a Rochellesalt crystal. It is mounted at one edge 1'! on the common supportingmember 13. The second or motor piezoelectric element 15 is provided withtwo foil electrodes 19 and 8|, as is well known. The motor elementelectrode between the adjacent faces of the two elements is grounded,and constitutes an electrode for the frequency controlling element 5. Amodulating voltage is applied between the two foil electrodes 19 and 8!with the result that a physical vibration is set up by the motor elementwhich varies the resonant frequency of the frequency controllingelement, and thus modulates the oscillator frequency.

While the illustration shows the grounded foil of the motor elementfunctioning as an electrode for the oscillating crystal, it is to beunderstood that separate elements may be used, if desired. That is, thetwo crystal elements may be located some distance apart and connected bysuitable coupling means, but, for simplicity, the arrangement shown ispreferred.

The present invention has been described by means of several specificembodiments, but there are, of course, many modifications which may bemade without departing from the spirit of this invention. Its scope,therefore, is only limited by the prior art and the spirit of theappended claims.

We claim as our invention:

1. In a variable air gap crystal holder, the combination including afixed electrode, a piezoelectric crystal mounted in a fixed positionwith respect to said electrode, a movable electrode adjacent saidcrystal whose position with respect to said crystal produces a change inthe frequency characteristic of said crystal, an armature, said movableelectrode being movable with said armature, armature driving meansresponsive to a modulating impulse, and means for increasing thefrequency change for a given movement of said movable electrodecomprising a condenser connected in shunt to said crystal.

2. The method of increasing the range of frequency modulation of avariable air gap crystal which includes the steps of utilizing saidcrystal to generate controlled frequency oscillations, varying the airgap of said crystal to modulate the frequency of said 5 oscillations,applying shunt capacitance across said crystal, and adjusting themagnitude of said capacitance to a value which substantially increasesthe frequency change for a given change of said air gap.

' ROBERT J. EHRET.

JOHN L. BARNES.

